1. Field of the Invention
The present invention relates to a curing agent for an epoxy resin. More particularly, the present invention relates to a curing agent for an epoxy resin which is widely used in casting, molding, laminating or coating or as an adhesive.
2. Description of the Related Art
An epoxy resin has various excellent properties such as adhesivity, processability and solvent resistance, and finds wide variety of applications.
With recent rapid progress of technology, it is highly desired for the epoxy resin to have much higher performances, in particular, heat resistance in various fields.
Methods for improving the heat resistance of the epoxy resin composition include improvement of the epoxy resin itself and improvement of a curing agent.
As the curing agent for the epoxy resin, amine compounds, acid anhydrides, phenol compounds and the like are known.
In general, when the amine compound is used as the curing agent, the cured epoxy resin has poor water resistance, and the amine compound is toxic.
The acid anhydride has a low curing rate, and the cured epoxy resin has poor water resistance and adhesivity.
In place of such curing agents, hydroxy novolak is used particularly in the molding material field since it has balanced heat resistance and water resistance.
With the recent progress of semiconductor technology, the phenol novolak type curing agent cannot impart satisfactory heat resistance to the cured epoxy resin in some applications.
For example, as an encapsulant for an electronic device such as an IC, a curing system in which glycidyl ether of o-cresol novolak is cured with hydroxy novolak is mainly used. Recently, a degree of integration of the IC increases and the ICs encapsulated by the epoxy resin compositions have been surface-mounted and directly submerged in a solder bath. The epoxy resin compositions for encapsulating electronic components are desired to have improved heat resistance.
As a curing agent for an epoxy resin which intends to impart good heat resistance to a cured product, various improvements of the phenol novolak type curing agents have been proposed.
First, it has been proposed to decrease a content of low molecular weight compounds, namely dinucleic compounds in the phenol novolak type curing agent or the epoxy resin.
For example, Japanese Patent Kokai Publication No. 212410/1987 describes that decrease of a content of dinucleic compounds in the novolak resin provides a thermosetting resin composition having better heat resistance and moisture resistance.
Further, the following Japanese Patent Kokai Publications intend to improve the heat resistance through decrease of the content of dinucleic compounds in the phenol novolak or the epoxy resin:
Japanese Patent Kokai Publication Nos. 98227/1981, 119220/1987, 301218/1988, 275620/1988, 225621/1988, 254123/1988, 81118/1988, 24818/1989, 64821/1989, 129055/1989 and 131273/1989.
As to the improvement of the heat resistance of curing agents for the epoxy resin, Japanese Patent Kokai Publication Nos. 198526/1983 and 22824/1988 disclose tris(hydroxyphenyl)methane and a condensation product of a phenol and salicylaldehyde.
The methods for decreasing the content of dinucleic compounds as disclosed in Japanese Patent Kokai Publication No. 212420/1987 and the like do not give a chemical structure which is substantially different from the conventional technique and the heat resistance is not sufficiently improved.
Since the improvement of heat resistance of the cured material of the epoxy resin usually increases a so-called called free volume, the moisture resistance tends to decrease and/or a coefficient of linear thermal expansion tends to increase.
The invention of Japanese Patent Kokai Publication No. 198526/1983 intends to improve the heat resistance through change of the chemical structure but the improvement is still insufficient and the moisture resistance is poor.
The invention of Japanese Patent Kokai Publication No. 22824/1988 does not improve the heat resistance and moisture resistance sufficiently and further improvement is desired.